This invention relates generally to braking of work machines, and more particularly to a drive train having a torque converter and an enhanced braking mode.
Heavy work machines, such as wheel loaders, off-highway trucks and scrappers, etc, can have relatively high momentum, even when moving slowly. Thus, wheel brakes are almost always insufficient to produce effective braking. Thus, many work machines also employ a device commonly known as a hydraulic retarder in conjunction with wheel brakes in order to generate the braking horsepower necessary for effective slowing of these work machines. Hydraulic retarders absorb vehicle momentum by stirring a viscous liquid, such as oil, when coupled to the work machine""s wheels. While hydraulic retarders can be effective, they are often a relatively expensive work machine component, and require additional cooling system capacity to reject the retarding energy.
A potential alternative to the use of hydraulic retarders is the use of engine compression release brakes. However, engine brakes often cannot produce braking horsepowers that are as high as that possible with the use of hydraulic retarders. Nevertheless, engine compression release brakes remain an attractive alternative when lower braking horsepowers are needed because they are generally substantially less expensive than hydraulic retarders and do not require additional coolers to reject the braking energy. Unfortunately, in those instances where specifications call for braking horsepowers beyond that capable with engine brakes, the only real alternative at this time is to use the relatively expensive hydraulic retarders.
Work machines of the type previously identified often include a torque converter position between the engine and the transmission. These torque converters sometimes include a lock up clutch that can be engaged to directly couple the engine output to the transmission input, or disengaged to allow the engine to be coupled to the transmission via the torque converter. It has always been the conventional wisdom to engage the lock up clutch when in a retarding mode in order to transfer as much wheel rotational energy from the work machine wheels to the engine, which is believed to be the dominant factor in a retarding mode that relies upon engine brakes. In other words, it has long been the conventional wisdom to engage the lock up clutch so that as much energy as possible can be absorbed by the engine via compression release brakes, which would be inherently less effective if the lock up clutch were disengaged.
Engineers have found that manipulation of the control of the lock up clutch can change the performance of one or more work machine components. One example of manipulation of a lock up clutch control to allegedly improve an aspect of performance is illustrated in U.S. Pat. No. 4,509,124, which issued to Suzuki et al. on Apr. 2, 1985. In this example, the lock up clutch is temporarily disengaged during gear shifting to reduce a shift shock which could be felt by the work machine operator. In addition to this use of the lock up clutch, engineers are always searching for other methods of improving work machine performance, especially when in a retarding mode.
The present invention is directed to overcoming one or more of the problems as set forth above, and to improving braking capabilities.
In one aspect, a method of enhanced braking comprises an initial step of coupling an engine to at least one wheel of a work machine at least in part via a torque converter. The torque converter is placed in an overspeed condition while activating an engine compression release brake system.
In another aspect, a drive train retarding system for a work machine includes an internal combustion engine having a selectively operable compression release braking system. A torque converter is operably coupled to the engine and has a locked condition and an unlocked condition. A braking enhancer is operable to place the torque converter in an unlocked condition when the compression release braking system is activated.
In still another aspect, a work machine includes at least one wheel and an engine with a compression release braking system attached to a chassis. A torque converter is attached to the chassis and positioned to operably couple the engine to the work machine wheel(s). A braking enhancer is operable to place the torque converter in an unlocked condition when the compression release braking system is activated.